1. Field of the Invention
The present invention is related to a starter having an epicycle reduction gear (planetary gear reduction device) for reducing rotation speed of a motor.
2. Description of Related Art
In a conventional starter described in JP-A-63-277859, an epicycle reduction gear is provided between a motor and a power shaft. An internal gear is used in the epicycle reduction gear. An outer peripheral section of the internal gear is rotatably provided along with an inner peripheral surface of a housing. A closed space is formed between a recess section formed in the outer peripheral section of the internal gear and a recess section formed in an inner peripheral section of the housing. An elastic block is provided in the closed space, so that rotation of the internal gear is restricted with respect to the housing via the elastic block. The elastic block is compressed and deformed when excessive torque works on the internal gear, so as to absorb impact force.
However, the closed space needs to be secured for providing the elastic block (cushion member) between the internal gear and the housing, in the above starter. Accordingly, the outer diameter of the housing is apt to be large, and the epicycle reduction gear becomes large.
Besides, a large cushion member cannot be used in this structure, because the cushion member is provided in the limited closed space. The cushion member has an area for receiving force when excessive torque works on the internal gear. However, in this case, the area of the cushion member is limited, and this structure cannot be used in a high-power starter.
Furthermore, this structure does not limit a compression degree of the cushion member. Here, as the torque applied to the internal gear increases, the compression degree of the cushion member becomes large. If the cushion member is repeatedly used in a large compression rate (e.g., over 30%), fatigue arises in the cushion member at an early state, and life of the cushion member shortened. As a result, the cushion member may be broken or damaged after a short period, and an impact-absorption capacity of the cushion member decreases.